Regarding the open neighborhoods
It was a design choice that have the game load one lot at a time. There is actually an explanation over the Sim Guru Tech Talk as far as I recall. Basically in one neighborhood objects on your lot are actually objects, meaning they are flagged to be used by the game and sims for autonomy usage. Everything is functional. owever objects on another lots aren't registered as objects but rather similar to fx. They cannot be interacted with, and since they are not considered objects by the game they are not being included with your sims autonomy and thus again since they are considered not an object but rather similar to fx, it saves performance.
Basically since objects in The Sims 4 are not like The Sims 3 where if you leave one lot the objects will dissapear, it made clear why they go to this route. Imagine if there are Fice lots in the neighborhood, all of them was composed of mansions built by the player with tons of objects and community lots. Since every object is loaded, all objects are candidate for autonomy and sims are complex in the game. The 20 sim limit will need to be removed, and each sims and onjects are capable of chaning emotions.
Some gameplay elements would also be removed. For example what about the Earthquake Lot Trait? They can't make one lot to only move while the rest is stationary.
They can bypass a lot of elements but it will need a huge amount of work, which make it seem impossible.
Alao the game currently suffers we all know from simulation lag. Making it an open neighborhood will make the game a lot more worse
From what I understand unlike The Sims 3 where they could easily create a world thanks to worlds being created from a third party program, in The Sims 4 they are hand crafted. If they are not then the world team would spend more time in creating neighborhoods, but as we all see, creating a neighborhood seems very hard to the dev team. Juat like SimGuruGrant said, sadly the way Terrain Editing works in our Lots are Not The Same off lot. They had to be handcrafted.
I am guessing they had a tool but since its EA dev tool they can't share it to the public. The only way we could build our own worlds is when the Dev Team created an external program for the players to use.
Meanwhile if you ask me, I would encourage them to create a tool right now for the players because it wouldn't just benifit us but also them. If they developed a tool, they can explore their world building program and engineer its limitaions.
(look at Island Living. Ocean swimming was impossible but thanks to the pack the engineers are able to make it possible. Its up to the producers if they are going to implement it in future/current worlds)
Once they now and bypass the limitations things would get more stable and smooth for both the studio and the community
